(a) Field of the Invention
The present invention relates to an apparatus for detecting the end of a signal-carrying groove of a record or video disk in a phonographic record player or the like.
(b) Description of the Prior Art
In the past, the detection of the end of a signal-carrying groove on a record disk has usually been performed by the detection of the difference in pitch between the signal-carrying groove and the lead-out groove. Devices are known for either mechanically or electrically detecting the difference in pitch of a record disk. In the mechanical device, the pickup arm, as it travels progressively toward the center of the record disk, progressively urges a lever of a means for detecting the groove pitch, i.e. the interval between the adjacently located portions of the record groove. When the pitch of traveling pickup arm exceeds a predetermined value for each one whole revolution of the turntable, the arm-returning mechanism is actuated. However, the pickup arm is subjected to a lateral pressure toward the outer peripheral direction of the record disk, so that the reproduced signal is distorted, causing the reproduced sound to become indistinct. In an extreme case, the stylus tip could jump out of the groove. Such jumping of the stylus tip is especially prominent when using a cartridge operable with a light vertical force. Furthermore, the groove-detecting apparatus of this type has a complicated arrangement, so that manufacturing must be precise to ensure proper performance. Thus, the designing and the adjustment of the apparatus are extremely difficult, causing an increase in the cost of manufacture.
As for the apparatus for electrically detecting the difference in groove pitch, there has been proposed an apparatus shown in FIG. 1 (Japanese Patent Publication No. Sho 48-16362). In the apparatus shown in FIG. 1, reference numeral 1 represents a pulse generator for generating pulses having periods inversely proportional to proportion, of the rotation speed of the pickup arm. Though not illustrated, this pulse generator is comprised of a shutter plate provided with a plurality of slits and arranged to rotate in accordance with the horizontal rotation of the pickup arm. Also provided are an optical mechanism including a light-emitting element provided above the shutter plate to face the slits section of this plate and a light-receiving element provided beneath the shutter plate to face said slits section. Finally a transducer converts a signal delivered from the light-receiving element into a binary pulse signal. During periods in which the stylus tip is tracing the record groove, pulses having long periods are outputted because of a small rotation speed of the pickup arm during the tracing period. When the stylus tip traces the end portion of the groove of the disk, pulses having relatively short periods are outputted because, during such time, the rotation speed of the pickup arm increases conspicuously. The output pulses of the pulse generator are counted by a counter circuit 2. This counter circuit 2 is reset periodically by an output of an oscillator 3. When the count of the counter circuit 2 has exceeded a predetermined value related to the period of oscillator 3, i.e., when the stylus tip traces the groove end portion, there is outputted a pickup arm returning signal to a returning mechanism actuating section 4, to thereby cause the pickup arm to return to the arm-rest position. It has been accepted generally that the signal-carrying groove has a pitch of about 0.1 mm, the lead-out groove has a pitch of 4.0.about.9.0 mm, and the pitch of the lead groove between adjacent two signal-carrying grooves is 1.0 mm which is not sensed by the groove-detecting mechanism.
In this known electrically-detecting apparatus, it is difficult, in view of the structure of the shutter plate, to form a number of slits in this shutter plate, i.e., the number of slits which can be formed is limited. Accordingly, it is not possible to reduce the period of the output pulses of the pulse generator 1 below a certain value. When the period of the output pulses is below a certain level, the precision of the detection is reduced so that an accurate detection can not be expected. Furthermore, to ensure high performance, the oscillator 3 must be highly accurate of low frequency. If oscillator 3 produces a high frequency level, there similarly appears a degradation of detection precision, so that a satisfactory detection cannot be expected either.
As discussed above, these two known types of groove-end detecting devices have been either expensive or low in precision of detection.